Ink jet printing is a non-impact printing method that propels droplets of ink onto a media such as paper or transparent film in response to an electronic digital signal. Thermal drop-on-demand ink jet printers have enjoyed wide acceptance as output for personal computers in the office and the home.
Three major concerns dominate ink jet ink technology: (1) the drying rate of the printed ink, (2) print quality, and (3) reliability. Drying rate determines the printing rate and, therefore, productivity of a printer. In a sheet fed printer, the ink must be dry before the succeeding sheet contacts it or the ink will smear. The drying rate can be increased by increasing the rate of evaporation of the ink vehicle, or by increasing the rate at which the ink penetrates the substrate on which it is printed, or both.
Print or image quality is the image sharpness and contrast versus the nonimage background areas and is determined by 2 factors: (1) Color properties of the ink which are measured by optical density and color coordinates controlling hue, e.g., L*, A* and B* in the CIELAB 1976 color standards; and (2) Non-color image characteristics which determine the definition of the image are: a) resolution, i.e., number of dots of ink per linear unit; b) edge acuity or sharpness; and c) presence or absence of print defects such as satellite droplets or feathering. In the case of black inks, the optical density is the most important of these two factors.
A major reliability issue is the result of pen nozzles becoming plugged due to precipitation or flocculation of the components in the ink. This property is measured by decap or crust time which is defined as the time span over which a pen can be exposed to the atmosphere without failing to print. Other reliability issues are stability of the ink caused by physical or chemical changes, compatibility with equipment material, robustness towards variability in thermal firing conditions and constant drop volume over long term use.
These concerns compete with one another in that improving one often produces a detrimental effect on the others. For example, using a more volatile cosolvent will increase drying rate, but also decrease decap time. A more penetrating cosolvent will also increase drying rate, but will also lower optical density and induce feathering. In addition, many of the known penetrant cosolvents destabilize pigment dispersions and readily block pen nozzles. Increasing optical density by employing more colorant adversely effects reliability.
Accordingly, a need exists for cosolvents that will increase ink drying without causing premature nozzle pluggage, without destabilizing pigment-based inks and without degrading print quality through feathering.